The first oracle at Delphi was commonly known as Sibyl, though her name was Herophile. She sang her predictions, which she recieved from Gaia. Later, "Sibyl" became a title given to whichever priestess manned the oracle at the time. The Sibyll sat on the Sibylline Rock, breathing in vapors from the ground and eating laurel leaves, gaining her often puzzling predictions from that. Pausanias claimed that the Sibyl was "born between man and goddess, daughter of sea monsters and an immortal nymph". Others said she was sister or daughter to Apollo. Still others claimed the Sibyl received her powers from Gaia originally, who passed the oracle to Thetis, who passed it to Phoebe.
The term can also apply to prophetesses from other oracles. The Cumaean Sibyl and the Erythraean Sibyl are especially notable.
Alternative: Pythia